1. Field of the Invention
The invention relates to an arrangement for mounting an optical element, in particular in an EUV projection exposure apparatus.
2. Prior Art
Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (=reticle) illuminated by means of the illumination device is in this case projected by means of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for EUV (i.e. for electromagnetic radiation having a wavelength of less than 15 nm), owing to the lack of availability of light-transmissive materials, mirrors are used as optical components for the imaging process. Said mirrors can e.g. be fixed on a carrier frame and designed to be at least partly manipulatable in order to enable a movement of the respective mirror in six degrees of freedom (i.e. with regard to displacements in the three spatial directions x, y and z and also with regard to rotations Rx, Ry and Rz about the corresponding axes). It is thereby possible to compensate, for instance, for changes in the optical properties which occur during the operation of the projection exposure apparatus e.g. on account of thermal influences.
WO 2005/026801 A2 discloses, inter alia, using in a projection lens of an EUV projection exposure apparatus for the manipulation of optical elements such as mirrors in up to six degrees of freedom three actuator devices each having at least two Lorentz actuators or two actively driveable movement spindles by means of which the respective optical element or the relevant mirror can be moved relative to the housing of the projection lens.
US 2008/0278828 A1 discloses, inter alia, fitting to an optical element in a projection exposure apparatus at least one additional element which brings about an energy dissipation of the vibration energy of the optical element by means of friction.
In order to transmit the drive force of an actuator to an optical element, rod-shaped components or pins are used, in particular, which can be effected for example in a known hexapod arrangement (see FIG. 7), in which each of a total of six pins or rods 10-60 has ball joints in its end sections in order to enable a decoupling of undesirable parasitic forces and moments during the actuation of the optical element (e.g. of a mirror M).
Between the baseplate 1 and the respective pins or rods, it is possible to provide an actuator for exerting a controllable force on the optical element or the mirror M. In the case of the small actuating distances (e.g. in the single-digit millimeter range), typically required in lithography applications, the use of solid-state joints in the form of universal joints has proved to be worthwhile for realizing the function of such ball joints, since such solid-state joints substantially exhibit a behaviour that is free of friction, without play and without hysteresis. Within the meaning of the present application, a universal joint should be understood to mean a joint which has two tilting joints having orthogonal orientation of the tilting axes with respect to one another (or tilting joints connected in series relative to the force flow), wherein said universal joints can have, in particular, a common pivot point.
In particular, a rod-shaped component or pin, for example in the arrangement from FIG. 7 (but not restricted thereto), can have two universal joints in its respective end sections for the purpose of decoupling in a lateral direction (i.e. perpendicular to the drive axis), such that the pin or rod has a high stiffness only in an axial direction for transmitting a force or movement, whereas only a low stiffness or a decoupling is present in all other directions.
FIG. 8 shows such a pin or rod 810 in a merely schematic illustration, wherein, of the universal joints 820, 821, only an individual joint in the form of a leaf spring is depicted in each case for the purpose of simplification, and wherein “801” designates a platform which carries an optical element such as e.g. a mirror and which is actuated by means of an actuator (not illustrated). In FIG. 8a-c, the stiffness of the linking of the pin 810 to the “fixed world” in each case by means of a spring is illustrated and designated by “815”.
What can be achieved given a suitable design of the pin 810 is that above the first axial natural frequency of the system composed of pin and actuator mass, disturbing natural frequencies of the mirror are filtered or suppressed. If a specific natural frequency in an axial direction is present, then excitations whose frequency exceeds this value of the natural frequency are coupled into the mirror in attenuated fashion in a manner dependent on the square of the quotient of this value of the natural frequency and the excitation frequency, since the system composed of actuator mass and pin brings about a corresponding 2nd order low-pass filtering. Even if this resonance is accompanied by an amplitude magnification, it is possible to obtain a stabilization in the control loop given a suitable design of the tuning parameters or choice of the suitable phase.
In this case, however, during operation the further problem can occur that undesirable transverse resonances, such as are indicated merely schematically in FIG. 8b and FIG. 8c, typically have significantly lower natural frequencies which, in the above numerical example, can typically be of the order of magnitude of 400 Hz. These transverse resonances are likewise accompanied by an amplitude magnification, wherein now, on account of the circumstance that the available or originally free tuning parameters have already been defined for taking account of the desired axial resonance described above, it is no longer possible under certain circumstances to prevent the control loop from becoming unstable.